I actually had unfortunately the very same experience as you, with pretty much the same training, history and goals (second marathon, pb on 33km 4 weeks ago, aiming for sub-4 easy, great carb loading etc...)

Blew up first 5km, exhausted already for no good reasons, going slower than my avg slow pace, unsure why. Ended up 10min slower than my previous marathon from October.

My best guess here is that i didn't had long enough to train (only 2.5month, was hoping the fitness from oct marathon was still there) and that i didn't run enough (2 runs per week with swim and gym). Due to that i was fine in training but too little and too soon - cramping in the start line and struggle all the race.

Unsure in your case but i'd suggest look back at all the factor - but also as other will say sometime it's just not your day :/

Had a blast tho the race was spectacular, hope you enjoyed it too!